IRAC: Extended Source Calibration

IRAC Photometric Corrections for Extended Sources

Extended (resolved) sources require special treatment to properly measure the corresponding integrated flux and surface brightness. The discrepancy between the (standard) point source calibration and the extended source calibration arises from the complex scattering of incident light in the array focal planes, particularly for the Si:As bands (5.8 and 8.0 µm). Our best current understanding is that the scattering has two related components, each of which contribute comparable effects: 1) the PSFs at 5.8 and 8.0 µm have more extended wings than expected, 2) there is a truly diffuse scattering (droop) which distributes a portion of the incident flux on a pixel throughout the entire array.

IRAC is calibrated using stars; this point-source calibration is applied to all IRAC data products to put them into MJy/sr units. The surface brightness of extended emission in these images will tend to appear BRIGHTER than it actually is. Photons that would normally scatter out of the PSF aperture used to measure a point source are instead captured by an extended source. To an extended source, the photons are not necessarily lost, but are redistributed. The scattering depends on the convolution between the IRAC PSF and how the light is distributed across the focal plane, which is usually quite complex for extended sources (galaxies, ISM & nebulae). To date, investigation of this scattering phenomenon has been conducted for Galactic HII regions, the ubiquitous zodiacal light, and elliptical galaxies. The analysis is ongoing, with only a partial (and somewhat unsatisfactory) solution that is presented in this page, reflecting our current understanding of the issue. Below are 'best practices' and 'aperture corrections' that we recommend to users of IRAC observations of extended sources. Please take note of the cautionary notes at the end of this page.

Best Practices for Extended Sources

Resolved galaxies with apertures centered on the nucleus:

• For sources < 8–9 arcsec in size, treat as point source (small aperture photometry, with local annular background subtraction)
  
• For sources > 8–9 arcsec in size, apply extended source aperture corrections (see below)

• If the source is small (compact), treat as point source (small aperture photometry, with local annular background subtraction)
  
  
• If the source is large & fuzzy, use the extended source aperture corrections (see below), beware that background structure will introduce large uncertainties (~10%)

• For very extended sources (> 300 arcseconds) or flat, low surface brightness sources (e.g., Magellanic-type galaxies), use the maximum scaling factors given below.

Be very conservative in interpreting colors as surface brightness measurements can be off by 5%–10% in the short-wavelength channels and 30% in the long-wavelength channels.

Extended Source Aperture Correction

The following aperture corrections are intended to correct the photometry of extended sources (e.g., galaxies) whose absolute calibration is tied to point sources. These corrections not only account for the "extended" emission from the IRAC PSF itself, but also from the diffuse scattering (droop) of the emission across the IRAC focal plane. The curves were derived from detailed analysis of elliptical galaxies (see related links at the end of this page). The curves may be applied to all types of galaxies, but beware that significant departures can be expected for sources that are morphologically different from elliptical galaxies (e.g., late-type LSB galaxies; see Surface Brightness recommendations above).

Extended source flux correction factors; solid lines represent exponential function fits to the data. Also indicated are correction factors derived from zodiacal light tests, and Galactic HII region tests (e.g. Martin Cohen's GLIMPSE vs. MSX, private communication).

Extended source flux correction factors for galaxies (solid lines) versus the PSF aperture correction factors (dotted lines). The main difference between the two is the truly diffuse scattering internal to the array.

Aperture photometry should also include background subtraction; we recommend that you use an annulus that is located just outside the boundary of your galaxy. Circular or elliptical apertures may be used.

The procedure for correcting extended source photometry is to apply the correction factor to the integrated flux measured from the IRAC image (subject to the standard or point source calibration). The correction factor is a function of the circular aperture radius or the effective circular aperture radius (if using ellipses). These corrections should be good to 10%. For convenience, we have converted the empirical curves into function form:

integrated flux (radius) = measured flux (radius) * correction_factor (radius)
correction_factor (radius) = true_flux / flux = [A * exp (-radius^B)] + C

where radius is in arcsec, A, B and C are best fit coefficients tabulated below:

IRAC	A	B	C
3.5µm	0.82	0.370	0.910
4.5µm	1.16	0.433	0.94
5.8µm	1.49	0.207	0.66
8.0 µm	1.37	0.330	0.740

The coefficient "C" represents the infinite, asymptotic value. Note that for IRAC-3 the curve continues downward from a value of 0.73 at R=200" to an infinite radius value of 0.66. However, it is not clear that the aperture correction should have this downward curvature or should flatten out by 200" radius (e.g., see the errorbars in aperture correction curves).

Low Surface Brightness Measurements and the Maximum Scaling Factors

Photometry of diffuse emission or low surface brightness objects is also subject to a large calibration correction in the IRAC 5.8 and 8.0 µm channels. The way to think about 'flat' extended objects is that any aperture you use to measure the integrated flux (or surface brightness) is equivalent to an infinitely large aperture applied to a point source (or galaxy). Hence, the appropriate aperture correction (or equivalently, surface brightness factor) is the large radius case of the above aperture corrections (but please see the cautionary notes):

IRAC	Surface Brightness Correction Factor
3.5µm	0.91
4.5µm	0.94
5.8µm	0.66-0.73
8.0 µm	0.74

Surface Brightness = measured surface brightness * correction_factor

Where the values represent asymptotic or infinite aperture values; note that for IRAC-3 the recommended correction is somewhere between 0.66 and 0.73, depending on the downward curvature of the aperture corrections (which is highly uncertain). These aperture corrections should be good to 10%.

Examples of LSB objects: large, late-type galaxies (e.g., NGC 300); Magellanic-type galaxies (e.g., NGC 6822); diffuse dwarf galaxies (e.g., M81 DwA); HII regions that are larger than ~100 arcseconds and not very centrally condensed.

Ongoing effort to characterize extended source photometry

• Extended PSF measurements
  
• Comparison of small elliptical galaxies to Palomar NIR measurements
  
• Comparison of HII regions to ISO Spectra/MSX
  
• Comparison of zodiacal light measurements to models
  

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